1. Field of the Invention
In the computer industry, hard disk drives have been used for many years for the permanent storage of data.
2. Description of Related Art
The need and thus the demand for more memory capacity and shorter access times on less memory space will necessarily result in a higher track density at a respectively higher speed. Higher track densities, in turn, can be achieved only with smaller track pitches and narrower data tracks.
With a track density of 25,000 tracks per inch (corresponding to a track pitch of approx. 1 xcexcm) with exponentially increasing tendency and a number of revolutions of 7,200 with linear increasing tendency, the requirements for the running accuracy of the spindle motors will also increase.
The drive motor, comprising a rotor with permanent magnet, a stator package with windings disposed on a base plate, a shaft, which is firmly connected with the rotor or the base plate, and a bearing system with at least one set of rolling bodies, causes the memory disk(s) disposed on the rotor to rotate.
In order to keep the effect of the unavoidable defects of form and component tolerances of the rolling bodies, the inner raceways and outer rings of the bearings on the running accuracy of the motor as low as possible it is common practice to brace the sets of rolling bearings against each other.
As a result, the bearing components rotating relative to each other are elastically deformed at their common places of contact. Consequently, each individual rolling body ball can be viewed as a system of two successively connected equivalent springs.
External forces in this spring-mass system lead the rotor to make evading motions or to wobble. As a result of an unfavorable frequency spectrum of the external forces the system can be caused to oscillate and to oscillate at a natural frequency. Depending on the system, the attenuation in this type of system is very low, so that the amplitudes of such oscillation may reach unacceptable values which could then lead to read/write errors.
Therefore, in order to further increase the running accuracy the external forces acting on the rotating system have to be minimized.
Said external forces are caused by electromagnetic interactions between the multipolar magnetized permanent magnet mounted in the rotor and the stator package. A sequential supply of current to the windings disposed on the stator package produces a magnetic traveling field that generates the torque as a result of the interaction with the individual poles of the ring-shaped permanent magnet, which torque drives the rotor. At the same time, forces acting radially on the rotor are generated.
Defects of concentricity and form can disturb the symmetry of said radial forces so that rotating or stationary residual forces can be generated which could cause the rotating system to oscillate and thus impair the running accuracy of the motor.
As a result of defects of form or position of the multipolar magnetized permanent magnet, which is manufactured by means of pressing and/or sintering and which is preferably plastic bonded and mounted in the rotor, and by positional errors (coaxiality errors, concentricity errors) of the stator package which is accommodated and positioned on the inside diameter via a collar-shaped projection in the base plate, the operating air gap of the motor is neither constant with regard to the width of the gap nor is it oriented concentric or coaxial relative to the axis of rotation. As a result the width of the gap also changes with the relative angular position between stator and rotor.
However, in order for the geometric sum of all the radial force components from the electromagnetic circuit acting on the rotor to be zero the width of the gap must be constant over the full circumference and independent of the angular position between rotor and stator.
In other words: the higher the positional errors of the stator and the higher the fluctuations in the gap width of the operating air gap that act on the circumference, the higher are the radial force components acting on the rotor in practical application.
This is where the invention comes into play with the objective of reducing both the deviation in the concentricity or coaxiality of the operating air gap with regard to the axis of rotation and the fluctuations in the gap width of the operating air gap over the circumference so as to minimize the resulting radial components of the external forces acting on the rotor.
Based on JP 10-248223 (1998), it is already known to center the stator with a mounting sleeve where the stator is pressed into the mounting sleeve and the mounting sleeve, in turn, is mounted on the stationary shaft. Said mounting is also achieved either with a press fit or by adhesive bonding.
This known embodiment, however, does not ensure that the required coaxiality of the operating air gap is achieved because the connection of the stator with the mounting sleeve is highly subject to tolerances which may cause a slanting position, for example, and the operating air gap is then no longer coaxial.
Another disadvantage of the above embodiment is that because the mounting sleeve is manufactured separately, the respective fitting surfaces of the mounting sleeve cannot be made with adequate accuracy and that a respective installation play will cause a mismatch when the individual parts of the spindle motor are assembled. As a result, the width of the operating air gap will vary in size and it is also dependent on the relative angular position between rotor and stator.
The same criticism applies to U.S. Pat. No. 5,925,946 which also provides a separately manufactured mounting sleeve which is connected with the stator so as not to rotate, which mounting sleeve, in turn, abuts on a vertically formed collar of the stationary base plate. A further disadvantage is that the centering does not take place on the actual relevant part, i.e. the stationary shaft, but only indirectly on the base plate which is connected with the stationary shaft.
Again, the potential mismatch could impair the coaxiality of the operating air gap.
Therefore, to solve the above problem the invention according to a first embodiment provides that the sheet iron package or the complete stator, consisting of sheet iron package and winding, in accordance with the invention is now encompassed by means of injecting a centering casing, which centering casing consists of plastic and where the centering casing eitherxe2x80x94in case of a stationary shaftxe2x80x94is centered directly on the stationary shaft, orxe2x80x94in case of a rotating shaftxe2x80x94it is centered on the outer ball bearing ring of the rolling bearing.
With the given technical theory of using a sheet metal package or a stator encompassed by means of injecting plastic which forms a centering casing that directly joins the stationary part thus offers the substantial advantage of minimizing the mismatch of the operating air gap, i.e. reducing it to a constant very low value. Normally, the size of such air gaps is 0.1 to 0.3 mm and applying the measures of the invention it is now possible for the first time to substantially minimize the tolerances in relation to the width of the air gap, which were previously viewed as restricting the function, so as to optimize the constancy of the air gap. It was possible to achieve a reduction of up to 3% in the mismatch which was caused previously according to the state of the art by dimensional tolerances.
The reason for this success, among others, is an injected centering casing which encompasses the sheet iron package and/or the stator. Using such an injected part has the advantage that the injection mould can be manufactured with high precision resulting in highly accurate, concentric and exact sleeve, supporting and positioning surfaces where the stator can be joined, centered and encompassed by injection which was not the case with the above mentioned separate mounting sleeves pertaining to the state of the art.
It is important that the sheet metal package, or the complete stator, is centered in the injection mold during the injection operation via one or more stop or supporting surfaces engaging on the outside circumference as the air gap tolerance is minimized according to the invention because the mismatch of the magnetically active outside diameter of the sheet metal package relative to the inside centering surface which is formed by the plastic mass is determined only by the accuracy of the injection mould.
Manufacturing the centering surface in the injection mould offers the further advantage that the work results achieved are always reproducible because the same form is always generated in the same place with the same injection and the same concentricity, which was not possible with the previously known separate mounting sleeves. The winding package is always placed in the injection mould at the same angular position.
Therefore, injection molding allows the reproducibility of the respective fitting surfaces, which was not the case in the state of the art.
Another characteristic feature of the invention is that the injected centering casing of the invention is directly put against the shaft that determines the axis of rotation of the rotor where it is centered precisely. This determines and provides a precise measure between the rotating permanent magnet or its inside surface forming the air gap and the stationary stator with its radially outside lying surface, irrespective of the relative angular position of the rotor.
Another embodiment of the invention provides that instead of a spindle motor with a stationary shaft a spindle motor with a rotating shaft is used. For this embodiment, separate protection is claimed independent of the above first solution. In this case, it is material that the injected centering casing is not centered on the base plate but analogous on the outer ring of at least one rolling bearing. The outer ring has the lowest possible mismatch in relation to the axis of rotation which is why it is preferred in accordance with the second proposed solution of the invention that the injected centering casing is put against and centered on said outer ring of the rolling bearing.
The minimal mismatch attributed to the stationary outer ring is the smallest possible mismatch of all stationary parts relative to the axis of rotation because it is connected with the shaft without play via the inner ring and the rolling bodies.
Therefore, the centering casing is directly associated with the virtual axis of rotation because intermediate members without play are interconnected, while leaving it open under this invention how said connection between said parts (stationary shaft or stationary outer ring of the rolling bearing) will be achieved. Both compression joining and adhesion bonding are claimed. In other words, the stator may be connected via a compression and/or adhesive connection with the respective stationary parts. Such stationary parts may include a shaft 2, a centering casing 19 and/or an outer ring 31.
In a third solution for solving the problem, it is claimed that instead of an injected centering casing a separate centering casing is used which was manufactured in advance, but which again is put against the stationary outer ring of the rolling bearing for centering purposes.
This type of centering between a separate centering casing which is connected with the stator and the remaining parts of a spindle motor has not previously been known according to the state of the art. Therefore, it is claimed as being relevant to the invention that said centering casing (whether injected or manufactured as a separate part) is put against and centered on at least one outer ring of a rolling bearing of the spindle motor.
Another embodiment of this third proposed solution provides that the two sets of rolling bodies which are disposed aligned one on top of the other rotate in a common outer ring and that the (injected or separately manufactured) centering casing is put against and centered on said common stationary outer ring of the rolling bearing.
In this case, it is also claimed as conforming with the solution if the complete centering casing is omitted and the stator merely lies directly on the outer ring of the rolling bearing of the spindle motor. Consequently, the centering casing is omitted completely in this case and the stator, preferably with its sheet iron package, is put against and mounted directly on the outer ring of the rolling bearing.
In reverse, when the centering casing and the stationary shaft are omitted the sheet metal package can be centered directly on the inner raceway of the ball bearing and/or the shaft.
The above mounting can also be achieved via various mounting mechanisms, such as compression, adhesion or wedging.
In a manner of speaking, the centering casing claimed in all previous embodiments is reduced to a zero value so that preferably the sheet iron package is directly put against the outer ring of the rolling bearing.
Also contributing to further solving the problem according to the invention, i.e. minimizing the mismatch for the benefit of optimizing the coaxiality of the operating air gap, in all previously proposed solutions is that high-precision permanent magnets are used. The radially inside lying surface of the permanent magnets defines the second surface of the air gap to be minimized.
In this case, it is preferable if the permanent magnets are mounted on the inside circumference of the rotating rotor so as to achieve the required coaxiality of the operating air gap. For attaching the permanent magnets in the rotor special centering devices are used which take measure based on the distance relative to the rotor bell and thus determine the place of installation for the magnet in the rotor with high precision.
The subject matter of this invention is not only the result of the subject of the individual patent claims, it is also the result of the combination of the individual patent claims.
All information and characteristic features disclosed in the documents, including the abstract, particularly the spatial configuration shown in the drawings, are claimed as relevant to the invention insofar as they are new individually or combined relative to the state of the art.